Add OpenWeatherMap weather forecasts. Closes #48

Show 8-day Bristol home weather on the index and weekends pages.
Show destination weather per day on the trip list and trip page.
Cache forecasts in ~/lib/data/weather/ and refresh via update.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

update.py

@@ -19,9 +19,11 @@ import agenda.geomob
 import agenda.gwr
 import agenda.mail
 import agenda.thespacedevs
+import agenda.trip
 import agenda.types
 import agenda.uk_holiday
 import agenda.uk_school_holiday
+import agenda.weather
 from agenda.event import Event
 from agenda.types import StrDict
 from web_view import app
@@ -406,6 +408,40 @@ def update_gandi(config: flask.config.Config) -> None:
         out.write(r.text)
 
 
+def update_weather(config: flask.config.Config) -> None:
+    """Refresh weather cache for all upcoming trips."""
+    from datetime import date, timedelta
+
+    today = date.today()
+    forecast_window = today + timedelta(days=8)
+
+    trips = agenda.trip.build_trip_list()
+    upcoming = [
+        t
+        for t in trips
+        if (t.end or t.start) >= today and t.start <= forecast_window
+    ]
+
+    seen: set[tuple[float, float]] = set()
+    count = 0
+    for trip in upcoming:
+        latlon = agenda.weather.trip_latlon(trip)
+        if not latlon or latlon in seen:
+            continue
+        seen.add(latlon)
+        lat, lon = latlon
+        try:
+            agenda.weather.get_forecast(
+                config["DATA_DIR"], config["OPENWEATHERMAP_API_KEY"], lat, lon
+            )
+            count += 1
+        except Exception as exc:
+            print(f"weather update failed for {lat},{lon}: {exc}")
+
+    if sys.stdin.isatty():
+        print(f"updated weather for {count} location(s)")
+
+
 def check_birthday_reminders(config: flask.config.Config) -> None:
     """Send at most one grouped birthday reminder email per day.
 
@@ -497,6 +533,7 @@ def main() -> None:
 
         agenda.fx.get_rates(app.config)
 
+        update_weather(app.config)
         update_thespacedevs(app.config)
 
         # Check for birthday reminders daily at 9 AM